The RAM type process involves pressing a quantity of plastic clay between cooperating male and female molds or dies formed of porous, fluid-permeable material. In addition to shaping the ware, the pressing operation also substantially dewaters the clay by forcing excess water into the pores of the molds. Release of the shaped ware, which adheres to the mold faces, is accomplished without distorting or damaging the ware by applying fluid pressure to a conduit embedded in one of the mold bodies so that the fluid passes from the conduit, diffuses throughout the porous mold body and exits through the mold face as a uniform blanket shortly before the male and female die members are separated. The shaped clay ware adheres to the second mold or die which is tranferred to a ware depositing station where fluid pressure is applied in a like manner to the second mold member to complete the release of the formed article. This basic process is disclosed in U.S. Pat. Nos. 2,584,109 and 2,584,110. In the pressure casting process, relatively liquid clay is used in place of plastic clay and the time within the mold is increased.
The original air-release mold bodies were formed of high grade gypsum plaster or gypsum cement which was found to have nearly ideal porosity for proper fluid permeability. Perforated metal tubing or permeable woven tubing was cast in the plaster molds to provide the required fluid conduits. The gypsum materials, however, were of limited hardness and consequently over the course of repeated pressing operations the faces of the molds would gradually wear away until the molds became unusable and had to be replaced. Although the service life of gypsum molds varied depending on the characteristics of the plastic clay being pressed, the configuration of the molds, the applied force and other factors, the practical service life of gypsum molds was generally no more than about 1,000 pressing operations. This necessitated relatively frequent replacement of the molds with the attendant disadvantages of expense for replacement molds, loss of production time, and nonuniformity of the produced ware due to slight differences between molds. Consequently, the ware forming industry searched for a substitute material for forming the molds which would have the required porosity characteristics closely approximating those of gypsum plaster and which would have a greater hardness enabling it to resist wear.
A crystalline bonded ceramic comprising at least 70% alumina, up to 15% ball clay and up to 15% talc fired to a point short of the theoretical density for the ceramic has been found to be a virtually ideal material. This material is disclosed in U.S. Pat. No. 3,384,499. The alumina, ball clay and talc composition with or without additives, such as manganese dioxide or carbon black, is formed into a slurry, cast and subsequently fired at a temperature ranging from 2,000.degree. F. to 2,350.degree. F. The exact firing conditions are controlled to prevent the ceramic mold body from reaching its theoretical density, i.e., the maximum density that the material would achieve if fired to an essentially solid nonporous state.
The necessary fluid conduit means were formed in the interior of the mold bodies by casting a combustible tubing in the interior of the alumina, ball clay, and talc mold body which was later consumed during the firing operation to leave an open conduit running through the mold body. It was generally considered necessary to form the fluid conduit in the interior of the mold body in order to provide for maximum transfer of fluid from the conduit to the mold body by utilizing the entire circumference of the conduit.
Porous fluid-release molds and dies formed from the new alumina, ball clay and talc material have vastly superior tensile strength, hardness, and wear resisting properties. Whereas a fluid-release, porous mold of gypsum material had a useful service life of approximately 1,000 pressing operations, molds and dies formed of the new material were capable of resisting wear and breakage and had a potential useful service life of literally tens of thousands of pressing operations.
In actual operation it was found, however, that the fluid permeability of molds or dies formed of the new alumina, ball clay and talc material progressively decreased so that after two or three thousand pressing operations, the molds were useless because they would no longer conduct sufficient fluid to effect a smooth release of the ware from the mold faces. After just a few thousand pressing operations, the new molds had to be replaced even though the mold faces were not appreciably worn or broken.
The problem of accumulation of residue in the pores of the mold bodies was solved by casting and firing the alumina, ball clay, and talc mold body without a consumable tube forming a conduit therein. After the mold body has been fired, fluid-permeable conduit means are affixed to the exterior of the mold body at some point other than on the mold face and the non-communicating surfaces of the permeable conduit means and the exterior surface of the mold body except for the mold face are sealed against passage of fluid so that when pressurized fluid is supplied to the fluid conduit means, the fluid is constrained to pass through the portion of the permeable conduit means communicating with the mold body and form thence through the mold body and to exit through the mold face. The size and spacing of the fluid conduit means may be vaired in order to regulate the egress of fluid through the mold face.
Such a conduit system is illustrated in U.S. Pat. No. 3,993,727 issued to Rudolf A. Skriletz and Virgil D. Kendall on Nov. 23, 1976, the disclosure of which is hereby incorporated. In this system, a mold body is first fired to a point short of the theoretical density of the material in a conventional manner specified in the '727 patent. The ceramic mold body is preferably comprised of at least 70% alumina, up to 15% ball clay and up to 15% talc by weight. The mold body is formed with a mold face on its exterior surface and a second surface on which the fluid conduit means is disposed. A groove can be formed in the second surface for reception of the fluid conduit means, or the fluid conduit means may be placed directly on an ungrooved second surface. The conduit means is a fluid-permeable conduit made of a perforated metal, a rigid metal mesh conduit or a tube formed of wooven fabric material such as cotton or nylon. Asbestos fibers could also be used for high temperature applications. Woven fabric is preferred because of its low cost and the ease of handling such light weight flexible materials. Perforated thermosetting semiplastic tubing or thermosetting plastic mesh may also be used.
After the fluid conduit is placed on the second surface of the mold body, it is sealed by covering it with a sealing composition. Practically any fluid impermeable adhesive resinous material may be used. The viscosity of the sealing composition should be sufficiently high that the sealer will not flow between the fluid permeable conduit and the mold body. The sealing composition should at least cover the entire length of the fluid-permeable conduit which is in contact with the mold body. Since the sealer material is relatively thick, it is applied by hand and frequently gaps are left over the fluid permeable conduit. Such gaps are difficult to detect by visual inspection. Hence, it is desirable that a second sealing composition be applied over the first sealing composition. The second sealing composition should be somewhat more liquid than the first sealing composition to flow into and seal any gaps which may exist in the first sealing composition. This second sealing material is preferaby spread over the entire second or outer surface of the mold body. Thereafter, a backing material comprised of an epoxy, sand and gravel mixture is applied. The mold body and backing material are all held in a metal casing.
The use of the fluid conduit covered by several layers of sealing composition is expensive both from a labor and a material standpoint. When this type of mold is used in a RAM process, the molds frequently deteriorate prior to unacceptable deterioration of the fluid conduits. The deterioration or failure of the molds in the RAM process is due to the high stresses placed on the mold and the abrasiveness of the clay. This is particularly true, if the mold face has a complex shape. Typically the ceramic mold has a life expectancy of 25,000 to 50,000 pressings. Typically the mold would be used a 1,000 times a day so as to have a life expectancy of two to three months. However, when the mold is used in press casting, the mold has a longer life expectancy, for example, ten years. This is true because less stress is placed upon the mold. A relatively liquid clay is placed in the mold, rather than the abrasive plastic clay. The mold is operated less frequently, for example, twice per hour. Also, the pressure applied to the mold is frequently less than the pressure which is applied in a RAM process. During such an expanded life span, it is expected that the tubing forming the fluid conduit would deteriorate prior to other failure occuring in the mold.